A New Multilayered Substrate for Facile Fabrication of Flexible One-Port Lamb Wave Resonator Sensors

While the recent trend of fabricating Lamb wave devices on polymer thick film realizes extremely bendable acoustic wave sensors, the excessive viscous damping of acoustic waves entering the polymer demands operation at relatively low device wavelengths (
$\le 10~\mu \text{m}$
). Consequently, the minimum feature size of these flexible devices is restricted to
$2.5~\mu \text{m}$
, which pushes toward the maximum resolution offered by contact lithography-based commercial ultraviolet (UV) mask aligners for batch fabrication of these devices. Accordingly, this work proposes a new multilayered zinc oxide (ZnO)/SU-8/ultrathin silicon (Si)/SU-8 substrate to fabricate flexible Lamb wave sensors operating at relatively large device wavelengths, thereby accommodating their facile patterning using low-cost UV-contact lithography. The proposed substrate attempts to reduce the resonant and nonresonant damping of Lamb waves by leveraging on the near-glassy nature of SU-8 polymer films at room temperature and employing a relatively low film thickness for the polymer. It was experimentally observed that the ZnO/SU-8/Si stack could be utilized to launch the S0 Lamb wave mode at
${K}^{{2}}$
in the range of 1.1%–1.3% while operating at device wavelengths
$\ge 70~\mu \text{m}$
. Further, a narrowband cavity resonance of the mode is excited using a one-port resonator structure with optimized geometry on the flexible multilayered substrate at a device wavelength of 80
$\mu \text{m}$
and tested for humidity sensing applications. The flexible resonator could provide a sensitivity of ~105 kHz/50% relative humidity (RH) and exhibit a similar trend for resonant frequency shift with changes in RH levels under both flat and bent states.